1. Field of the Invention
The invention relates to an additive combination of an oil-soluble aliphatic copolymer having the property of a nucleator for wax crystallization e.g., an ethylene-vinyl acetate copolymer, with an oil-soluble hydrocarbyl substituted succinic acid which has been reacted with a nitrogen material to form a derivative thereof. This combination is particularly useful in distillate fuel oil having a final boiling point above about 370.degree. C. and also having an undesirable property of supercooling which reduces the effectiveness of wax growth arresters in preventing the formation of large wax crystals.
2. Description of the Prior Art
Middle distillates containing normal paraffins have the characteristic of losing their fluidity rather suddenly as the temperature of the oil is decreased. This loss of fluidity is due to the crystallization of the normal paraffins into plate-like wax crystals which eventually form a spongy mass entrapping the oil therein.
Generally, the process of crystallization occurs through supercooling of a liquid phase which can be either a melt or a solution. The degree of supercooling necessary for onset of crystallization varies considerably depending on the nature of the system. In order to arrest the growth of wax crystals in petroleum fractions, the arresting is conventionally accomplished by use of additive compounds defined as wax growth arresters. The ease or facility of arresting the growth of wax crystals is functionally inversely related to the magnitude of supercooling of the petroleum fraction before onset of crystallization, i.e. the closer crystallization occurs to the saturation temperature, the more effective will be the growth arrester. For this reason, one seeks to discover compounds which exceptionally reduce said magnitude of supercooling, as nucleators for wax crystallization thereby facilitating the function of the growth arrester. These nucleation and growth arresting phenomena must not be confused with pour point depression.
In addition to this phenomenon described above the fluidity of distillates is impaired by their tendency to congeal due to wax separation. This undesirable characteristic is overcome by the use of compounds referred to as pour depressants which function by being absorbed on the surface of the wax crystals and in so doing prevent oil gelation. The present inventive disclosure of this application does not relate to this aspect of low temperature fluidity of waxy distillate fuels.
It is also known that wax crystal modification is a variable phenomenon depending upon the particular petroleum product to be treated. Thus, the molecular size and structure of waxes in a particularly defined petroleum product will vary considerably, depending upon the type of crude oil and refining process utilized to arrive at that product. The effectiveness of the particular wax modifying material will vary from petroleum product to petroleum product.
The difference in the ability of a particular wax crystal modifier to affect a petroleum product is often referred to as the responsiveness of the treated product to the modifier. Thus, some petroleum products are much more responsive to wax crystal modifiers of a given type than another petroleum product would be to the same wax crystal modifier.
Moreover, wax crystal modifiers were generally considered to be polymeric materials, such as the classic ethylene vinyl acetate copolymer.
Recently, it has become known that pour point depression alone is not a sufficient phenomenon to alleviate some problems caused by wax crystals in various fuels, especially middle distillates. In those petroleum fractions, it has been observed that the wax crystals formed in the presence of the pour point depressant are often too large to enable the wax-cloudy fuels to pass easily through screens and orifices commonly encountered in the equipment employed either in distribution or in use of such fuels. This problem has been alleviated by the addition to said fraction of petroleum products of wax crystal modifiers which are referred to as flow and filterability improvers.
U.S. Pat. No. 3,961,916 teaches that the low temperature flow characteristics of petroleum middle distillates can be very satisfactorily controlled by the proper choice of a combination of a nucleating agent or wax growth stimulator and a wax crystal growth arrester. This was based on discovery that depending on its composition and physicochemical characteristics, such as molecular weight and branchiness, a polymeric wax crystal modifier can essentially operate as a nucleator or as a growth arrester for wax crystals. According to this patent, one convenient way of achieving this in a most effective way is to add a separate polymeric additive to effect each of the separate functionalities desired.
Another disclosure in the prior art is that of U.S. Pat. No. 3,444,082 which teaches that a combination of alkenyl succinamic acid and the amine salts thereof with ethylene copolymers are good for reducing the pour point of various petroleum fuels. These ethylene polymers are in preferred form said to contain polymethylene segments separated by a divider resulting from the presence of a comonomer copolymerized with the ethylene. Those comonomers include hydrocarbon terminal olefins of from about 3 to 12 carbon atoms and various heteroatom containing addition polymerizable terminal olefins such as the acrylates, methacrylates, vinyl ethers, vinyl ketones, vinyl esters, etc. (see col. 4, 11. 48-49 and Table IV).
Further, U.S. Pat. No. 3,850,587 teaches a three component flow-improver admixture for waxy hydrocarbonaceous fuels comprising: (1) a C.sub.8 to C.sub.28 hydrocarbyl succinamic acid mono- or disubstituted on the nitrogen atom with C.sub.8 to C.sub.28 hydrocarbyl groups; (2) an ethylene-vinyl acetate polymer containing from 10 to 40 weight percent vinyl acetate and having a molecular weight between 800 and about 10,000, and (3) an aromatic acid having from 7 to 20 carbons whereby the cold flow properties of distillate fuels are improved.